1. Field of the Invention
The present invention relates to a hydraulic control circuit for an automatic transmission, particularly to a hydraulic control system having a neutral control for a hill-holding stop position a neutral control for high speed coasting, and a protective control against reverse.
2. Description of Prior Art
An automatic transmission, in general, causes creeping which makes an automobile run forward due to the so called "dragging torque" of a torque converter in case that an automobile is stopped by a manual valve in the D range. A driver usually needs to step on a brake during the entire time an automobile stops.
Under such circumstances, a control device has been proposed to avoid creeping from low torque in the high speed range which is automatically shifted when an automobile stops, to ease fatigue of the driver and increase fuel efficiency. Another proposed device avoids creeping by disconnecting the forward clutch so that torque is not transmitted to the wheels. On the other hand, shifting to the high speed range can avoid creeping under normal idling condition but still produces creeping at high speed idling during warm-up of the engine.
Furthermore, disconnecting the forward clutch requires an accumulator, modulator valve, solenoid valve and other special equipment to prevent shift-shock when said clutch is connected. Because of the above, the structure becomes complicated and big, the cost of said device increases and problems on reliability occur. In addition, if the forward clutch is disconnected, no restriction exists on the vehicle, so that it may start rolling even on a very small incline. To avoid this, it is desirable for this type of device to incorporate a hill-holding mechanism. However, this results in a more complicated structure.
During high speed coasting where no engine torque is required, when an accelerator is released, an engine brake is applied slightly in the D range. So, to obtain high fuel efficiency, it is desirable to disconnect the clutch automatically during high speed coasting. And incorporating a mechanism which prevents an automatic transmission from going into the reverse condition even when the shift valve is shifted to the R range, particularly when the vehicle is running faster than a certain speed helps enhance safety.
Accordingly, mechanisms for neutral control during high speed coasting and preventing R range shifting due to errors in operation, in addition to mechanisms for anti-creeping and hill-holding, cause "locking" of automatic transmission during high speed running and require actuators and valves to control the system, resulting in diminished reliability, cost increase and an excessively large system.
An object of the present invention is to provide a hydraulic control system for an automatic transmission which has the following features with a very simple structure:
(1) an anti-creeping mechanism; PA0 (2) a hill-holding mechanism; PA0 (3) neutral control during high speed coasting; and PA0 (4) preventing R range shifting during erroneous operation. PA0 first port (.mu.)--connected to forward clutch hydraulic servo (C.sub.1); PA0 second port (.beta.)--connected to brake hydraulic servo (B.sub.1); PA0 third port (.alpha.)--connected to an oil path provides line pressure; and PA0 fourth port (.delta.)--connected to modulator valve (41).